Slag cement is conventionally made of vitreous blast furnace slag (hereinafter referred to as slag) such as water granulated blast furnace slag. A disadvantage of such slag cement is its slow hydraulic hardening in the early stages. In order to improve the hydration setting of cement, a technique of using finely milled slag is known. However, conventional slag milling techniques provide a maximum Blaine specific surface area of only about 4,200 cm.sup.2 /g. This is because upon milling to a high fineness, milling efficiency is reduced and the milling cost is very high. More specifically, as slag milling progresses, secondary particles form; the specific surface area does not increase much even if the milling time is prolonged. As a result, when slag is milled finer, milling efficiency decreases. This tendency is particularly noticeable when open circuit milling (to be described later) is performed. When slag is milled to a high fineness using milling equipment such as a ball mill, surface activity is expected to decrease due to the mechanochemical effect. Hence, due to losses in time and energy, slag is not in practice milled to a high fineness.
Although it is not a milling technique specifically developed for vitreous blast furnace slag, a technique is known for milling clinker sintered in a kiln and a small amount of gypsum. This technique is classified into open and closed circuit milling techniques.
In the open circuit milling technique, only a mill is used; and a separator is not used to obtain a product. This technique, however, can easily cause overmilling of the cement in the mill and broadening of the particle size distribution of the product. In addition, the milling temperature is increased and the cement quality may suffer.
In the closed circuit milling technique, after raw materials are milled in a ball mill, the milled powder is classified with a separator to produced fine powder. Unlike open circuit milling, this method does not require milling to a high specific surface area. With this method, therefore, the electric power unit of the overall milling system is lower than that in open circuit milling. Sturtevant and cyclone separators are most frequently used in closed circuit milling. In a Sturtevant separator, fine powder circulates within the separator. Therefore, classification efficiency is low and the product has too broad a particle size distribution. In a cyclone separator, fine powder is trapped by a cyclone. Classification efficiency is high, and the product has comparatively narrow particle size distribution.
In the manufacture of cement by closed system milling, classification is performed only as an auxiliary milling process, and classification at a specific cut size is not performed. Cement actually produced by a classifier has a Blaine specific surface area of about 3,600 to 4,200 cm.sup.2 /g. In the manufacture of cement by closed system milling, classification of a specific cut size is not performed for the following reason. Portland cement used to make clinkers has different levels of ease in milling in accordance with constituent minerals. If the milled clinker is classified, the mineral composition changes, therefore, classification is not preferred in this case.